1. Field of the Invention
This invention relates to continuous stream type ink jet printers, and more particularly, to such printers having a printhead with integral electrohydrodynamic electrodes and nozzle plate.
2. Description of the Prior Art
Ink jet devices of the continuous stream type generally employ a printhead having a droplet generator with multiple nozzles from which continuous streams of ink droplets are emitted and directed to a recording medium or a collecting gutter. The ink is stimulated prior to or during its exiting from the nozzles so that the stream breaks up into a series of uniform droplets at a fixed distance from the nozzles. As the droplets are formed, they are selectively charged by the application of a charging voltage by electrodes positioned adjacent the streams at the location where they break up into droplets. The droplets which are charged are deflected by an electric field either into a gutter for ink collection and reuse, or to a specific location on the recording medium, such as paper, which may be continuously transported at a relatively high speed across the paths of the droplets.
Printing information is transferred to the droplets through charging by the electrodes, the charging control voltages are applied to the charging electrodes at the same frequency as that which the droplets are generated. This permits each droplet to be individually charged so that it may be positioned at a distinct location different from all other droplets or sent to the gutter. Printing information cannot be transferred to the droplets properly unless each charging electrode is activated in phase with the droplet formation at the associated ink stream. As the droplets proceed in flight towards the recording medium, they are passed through an electric field which deflects each individually charged droplet in accordance with its charge magnitude to specific pixel locations on the recording medium.
A common method of perturbating an array of continuous ink jets is by a piezoelectric driver. The driver produces acoustic waves which traverse an ink reservoir to the nozzles, perturbating the jets and ideally causing uniform breakup of the jets in terms of break-off length and phase. Thus, the drop generator reservoir or manifold has two functions, to distribute ink to the individual nozzles and to distribute acoustic energy to the individual jets to cause a controlled uniform breakup into droplets.
In practice, there are a number of difficulties associated with this approach, most of them related to the manifold or reservoir. Since the reservoir is an acoustic pathway to the jets, it must be acoustically designed. This means the materials used should be acoustically matched to the ink and the fabrication must be of high precision. The completed drop generator must have a piezoelectric driver accurately positioned in a precision reservoir which also must have a precise array of nozzles. The droplet generators successfully meeting the design criteria tend to be quite bulky and heavy, costly to fabricate, and when used in a carriage type configuration, place a stressing burden on the carriage mechanism.
To eliminate the problems associated with printheads having acoustic reservoirs that distribute acoustic energy from the piezoelectric driver to the individual streams of ink emitted from the nozzles, electrohydrodynamic electrodes may be positioned at the printhead nozzles or orifices, or certain forms of thermal energy pulses may be used to perturbate the streams and cause the uniform breakup of the streams at fixed distances from the nozzles.
U.S. Pat. No. 3,878,519 to Eaton discloses the selective application of heat energy to the ink stream emitted under pressure from a nozzle to reduce the surface tension of successive segments of the ink stream before the ink stream would randomly break up into droplets. Both the quantity of energy applied and the duration of the applied energy, control the breakup point of the stream at predetermined distances from the nozzle. The source of heat may be high intensity light converted to heat energy by the ink stream, or by an annular or partially annular resistive heater positioned with the nozzle and at the nozzle orifice outer surface.
U.S. Pat. No. 3,596,275 to Sweet discloses the basic concept of an EHD exciter. The disclosed electrohydrodynamic (EHD) device requires very high voltages and expensive transformers to obtain them. The high voltages represent an electrical complexity, high cost, and safety hazard. The high voltages needed to excite or pulsate the fluid column also interferes with the subsequent droplet charging step.
U.S. Pat. No. 3,949,410 to Bassous et al discloses an EHD exciter integrated into a nozzle. In connection with FIG. 4, they describe the fundamental EHD process first articulated by Sweet in his above-mentioned patent. Bassous et al report the periodic swelling and non-swelling of a fluid column due to the electric field associated with the geometry at the nozzle orifice. They further disclose the fluid mechanics principle that the wavelength of the swelling (i.e. droplet separation) is given by the velocity of the fluid divided by the frequency of the swelling or perturbations.
U.S. Pat. No. 4,220,958 to Crowley discloses a continuous stream-type ink jet printer wherein the perturbation is accomplished by electrohydrodynamic excitation. The EHD exciter is composed of one or more pump electrodes of a length equal to about one-half the droplet spacing. The multiple pump electrode embodiments are spaced at intervals of multiples of about one-half the droplet spacing or wavelength downstream from the nozzles.
U.S. Pat. No, 4,047,184 to Bassous et al discloses a charging electrode array for use in continuous stream type ink jet printers that is formed by anisotropic etching of apertures through a silicon substrate. Conductive diffusion layers in the walls of the apertures permit charges to be placed on the drops at the point of break-off from the ink streams as they pass through the apertures. In one embodiment, the printer nozzles emitting the ink streams are combined with the charging electrodes.
U.S. Pat. No. 4,343,013 to Bader et al discloses a nozzle plate for an ink jet printhead. The front surface of the nozzle plate and the area around the nozzle orifice is provided with a non-wetting coating or material with respect to the ink comprised of water-repellant metal or plastic. The nozzle plate is glass and the nozzles are produced therein by a photoetching process. The front side (downstream side) of the nozzle plate is coated with such water-repellent material as chromium, nickel or Teflon. Such a coating prevents deposits of ink at the front surface around the nozzles.
U.S. Pat. No. 4,555,062 to You discloses an ionic surface preparation for the nozzles of ink jet printers. The front of the nozzle plate and the surface of the nozzle are ionically activated so that the surface is able to selectively adsorb some of the anti-wetting compound added to the ink. If the desired anti-wetting compound is anionic, the nozzle surfaces are pretreated with a cation. In the case of a cationic anti-wetting compound, the surfaces are pretreated with anions. The pretreatment method is primarily dependent on the nature of the material used to produce the nozzle. P-type ions such as boron can be implanted if the nozzle surface is silicon dioxide. If the nozzle surface is a metal such as nickel, ions such as chromium can be applied by wet chemistry. A typical long chain anionic non-wetting agent such as FC-143 available from the 3M Company of Minneapolis, Minn. is then dissolved in the ink for subsequent adsorption by the pretreated nozzle surface area.
U.S. Pat. No. 4,560,991 to Schutrum discloses an electroformed charge electrode for a continuous stream-type ink jet printer. The charged electrode structure comprises a dielectric substrate having a plurality of spaced electrodes embedded therein. Electrically conducting circuit leads are embedded into a second surface of the dielectric substrate and connect to the electrodes. The electrodes can thereby be charged by a voltage source.
U.S. Pat. No. 4,568,946 to Weinberg discloses a charge electrode means for sensing a charge on the individual ink droplets passing thereby and providing signals which can be used to control the timing of the charging electrical pulses applied to the charging electrodes. The charge electrode means comprises a pair of electrical insulating members mounted in spaced relation to one another so as to provide a gap between opposed surfaces thereof. Conductive charge electrode layers are provided on these opposed surfaces and are electrically connected.
In EHD stimulation of synchronous ink streams, an electrode is generally placed in the proximity of the stream a short distance downstream from the nozzle. This electrode is biased by a time varying voltage in respect to the ink stream, and hence, it has to be electrically insulated from the ink by, for example, a dielectric spacer. The distance from the beginning of the ink stream as it exits from the nozzle to the EHD electrode is defined by the dielectric spacer. The dielectric spacer has to function as an insulator in a hostile environment, being exposed to ink vapor, ink mist, and ink contamination during startup and shutdown of the ink stream. In prior art EHD stimulated continuous stream ink jet printing, the resistance between the ink stream and the electrode was found to be too low for successful drop generation. Such EHD stimulated ink jet printers often shorted with a long recovery period after startup, and it was found to be time dependent with the streams running during printing operation. The cause of the above-mentioned problem was found to be the ink wetting the dielectric spacer. The spacer surface contained microasperities and these microasperities cause the wetting even if the spacer material was non-wettable by the ink.